1. Field of the Invention
The present invention generally relates to impact printers and, more particularly, to operator control of print head to platen or stock spacing, particularly in connection with arrangements for automatic compensation for stock thickness.
2. Description of the Prior Art
The increase of use of data processing systems and personal computers, particularly by businesses, has been accompanied by increased demand for printers capable of providing a high quality type font at high speed. Numerous technologies have been investigated for simultaneously answering these two requirements. However, current business practices often involve the need to print multiple copies at the same time on layered stock and not all printer technologies are suited to such requirements.
Such multiple copies, which are often color-coded for regulating distribution and communication of respective ones of the copies, are often preferred because of the assurance they provide of exact duplication of the information printed and for the convenience of uniformity of the number of copies and the ability to establish procedures for handling of each copy. These attributes are relatively difficult to duplicate with printers using, for instance, laser and ink-jet technologies since these technologies do not provide a mechanism for producing an image on other than the surface layer of multi-layer stock. Data processing techniques for generating multiple, serial copies has generally been limited to automatic generation of labels, often printed in the margins of documents. Even then, the very flexibility of data processing systems does not assure that the same number of copies with the same use, distribution or disposition designations will be uniformly produced. The production of color-coded copies, which may be easily accomplished with layered stock, requires the maintenance of inventories of multiple paper stocks and, often, the manual feeding of these different stocks to the printer unless complex and costly sheet feeders are employed.
Printer technologies which are capable of forming images on each layer of multi-layer stock generally rely on impact forces which may be transmitted through all sheets of the stock. Some such printers use technologies which are outgrowths from the typewriter arts such as so-called type ball and daisy wheel printers. Such technologies develop full "letter quality" but are limited in the number of characters and fonts which can be produced without manually changing the type ball or daisy wheel. So-called band printers are similarly limited. To produce a greater number of characters, symbols and fonts in a variety of symbol point sizes and pitches, so-called pin printers have gained widespread popularity and have developed resolution capabilities (e.g. dots per inch) which allow print quality to approach that of laser and ink-jet printers at the level of human visual perception. For purposes of this disclosure, these technologies (e.g. type ball, daisy wheel, band and pin printers) will be generically referred to hereinafter as "impact" printers.
Impact printers are well-known and are in widespread use at the present time. Being principally reliant on mechanical action of a relatively limited number of parts in the print head and transport therefor, they are generally less expensive than printers using other technologies. Further, while the actual printing action is far slower than in comparable laser or ink-jet printers, the mechanical constraint to lower image dot pitches reduces the amount of time required for "spooling" or the mapping of symbol codes to a dot image or character map from which the printer is driven. Therefore, overall printing time is comparable and may be less than that of laser and ink-jet printers, particularly on printed forms where relatively few characters or symbols are to be formed. (In contrast, ink-jet and laser printers typically form a dot image of the entire page or form prior to printing.) Accordingly, impact printers remain preferred for many applications, even where printing on multi-layer stock is not required.
Due to the mechanical action of impact printers, the print head to platen spacing is relatively critical to the print quality produced, especially in regard to the stock on which printing is done. The spacing between the print head and the platen or the surface of the paper stock relative to the distance over which the pins or type are accelerated greatly affects the impact forces which are applied to the stock. The optimum velocity is also subject to numerous other printing variables and parameters. For example, most impact printers include a ribbon for applying ink to the stock or the uppermost layer thereof and the forces applied thereto affects the efficiency with which ink transfer to the stock takes place. The mechanical motion of the ribbon, the amount of ink carried thereby and the texture of the paper are only a few of many other conditions which affect print quality and require relatively close regulation of pin or type velocity to obtain results which are considered satisfactory at the present state of the art. Therefore, it is common practice at the present time to at least provide manual adjustment of the print head to platen distance in order to allow for near-optimization of the print quality for different stocks.
An arrangement for automatic print head spacing adjustment to accommodate a plurality of paper stock and form thicknesses (hereinafter sometimes referred to collectively as "sheet material") with which the present invention is preferably implemented is disclosed in U.S. patent application Ser. No. 08/011,460 by Campbell et al., filed Jan. 29, 1993, which is assigned to the assignee of the present invention and hereby fully incorporated by reference. A summary of a prior arrangement (which is specifically not admitted to be prior art as to the present invention) is also described therein with which the present invention may be used. While this arrangement produces high quality print over a wide range of sheet material thickness and even a variety of textures where texture can be identified with particular thickness, the number of variables in an impact printing process almost necessarily requires some degree of operator intervention in order to obtain optimum results. Consider, for example, that a relatively new ribbon used in an impact printer will have a relatively high efficiency of transfer of ink to sheet material regardless of sheet material texture. A rough textured paper or a form of many layers may be directly smudged by a relatively new ribbon even at locations where printing is not done unless the head gap is increased from a gap which would otherwise be optimum. As the printer is used and the amount of ink in the ribbon is reduced, ink transfer efficiency is also reduced. However, ink transfer efficiency will not be reduced at the same rate with continued printing for both rough and smooth textured surfaces. Near the end of the useful lifetime of a printer ribbon, satisfactory printing may be possible on a rough textured stock but not on a smooth textured stock or vice-versa. Finally, the operator may wish to significantly increase impact forces to extract all possible ink (and printed pages) from a ribbon before it is discarded, even at the expense of poor print copy. Therefore, even when automatic head gap setting is provided and which provides superior results during the great majority of operating conditions and printing condition variables, the results may usually be further improved if the operator or user is allowed to supplement the action of the automatic system.
However, providing for such operator intervention in a manner which will accomplish improvement of results is complicated by many practical matters. For example, the range of such an adjustment is on the order of 0.010 inches or .+-.0.005 inches in 0.0005 inch increments from the position which would be set by an automatic head adjustment. A separate manual adjustment system which would perform in this manner and also provide repeatability would necessarily be complex and expensive. Further, any such separate system would interfere with the function and accuracy of any automatic system provided in the printer. Likewise, adjusting the print head gap by an electrical control arrangement would interfere with the operation of an automatic head gap adjustment arrangement as well as being largely duplicative thereof. Attempting to combine a user-controllable arrangement with an automatic arrangement raises the issue of separating the automatic and user-controllable functions (e.g. the user-defined values may cause loss of the automatic set point values supplied by the printer manufacturer).
Further, when the printer is being used to print on alternate sheet materials or paper stocks such as bond paper and multi-layer forms, different adjustments may be necessary or desirable for each. For example, bond paper may require an increase of head gap from a set point determined from an automatic system due to ribbon condition while a multi-layer form may require a decrease from an automatically determined set point in order to form satisfactory images on all sheets thereof. If these sheet materials are alternated, an adjustment will be required each time the sheet material is changed. Accordingly, it is desirable to be able to recall user defined set points in the same manner as manufacturer supplied automatic set points.
As a practical matter, whether the adjustment is made mechanically or electrically, most known head gap adjustment arrangements require opening the printer cabinet if not an off-line self-test or set-up routine. Accordingly, the result of an adjustment can only be observed by the printing of an entire page, which is wasteful and time consuming. Currently available printers do not allow adjustment while printing is being done on-line. Therefore, there is a time lag between the time the adjustment is made and the time the effect of those adjustments can be seen by the user. Therefore, there is no "feedback" to the user as the adjustment is made, reducing accuracy of the adjustment made and increasing the number of times adjustment must be made to arrive at a result which the user considers to be satisfactory.